The current carrying capacity of conventional circuits employing hybrid technology is controlled by the thickness of the screened metal layers used therein and by the necessity of restricting the number of successively printed patterns due to material limitations. In order to improve the current handling capacity, some methods were developed in which a combination of thick film and copper plating techniques are used.
In general known methods for producing hybrid circuits capable of handling heavy currents employ three basic steps: (1) the application of a conductive seed layer using thick film screening techniques, usually noble metal based systems, or using thin film techniques, as by sputtering metal directly onto the ceramic or by ion implanting metal onto the ceramic; (2) copper plating of the seed layer; and (3) etching of the desired pattern using techniques similar to those used in the manufacturing of printed circuit boards.
In particular three methods used to implement such steps having a trade description are: ENPLATE (by Enthone, Inc. of West Haven, Conn.), CERACLAD (by Kollmorgan Corp. of Hicksville, N.Y.), and COPPER-ON-CERAMIC (by ICI of Los Angeles, Calif.). The differences between the above mentioned methods occur mainly in the seed layer material which is used. In all these techniques a substractive method (i.e. a method which requires the use of etching) is used to define the conductive traces, which method uses expensive materials (for the etching process) and extra labor (for the number of production steps). Moreover, such techniques do not provide for the possibility of creating high integration level, multilayer circuits, meaning that the density of the circuits is very low. Further the COPPER-ON-CERAMIC method provides crossovers with heavy current capacity on the top layer only. Theoretically it is possible to take these single layer technologies and create crossovers by using known methods developed for copper thick film technology using an oxygen doped nitrogen atmosphere. However, in practive it is too expensive a technique to use for such a purpose.
In conventional hybrid circuits for high current applications, one additional method is known, often referred to as the "successive prints" method. For such successive printing operations, a circuit must normally be realigned to the previously printed layer to within 0.002" or electrical shorting may occur between adjacent areas. This alignment accuracy tends to increase the cost of manufacture and further requires extra labor to be involved in the method as well.
Crossovers are also critical in heavy current high density hybrid circuits. To create conventional thick film crossovers for such circuits usually three structures are used: (1) a continuous screened conductor with a dielectric layer in between, (2) a structure in which one of the two crossed conductor lines is interrupted and then connected again with a wire above the other conductor (a so-called wire bonded crossover in which a dielectric encapsulation is usually placed underneath the bond to insulate the wire bond from the conductor being crossed); and (3) a structure which is the same as structure (2) but where, instead of a wire, a copper bar is used to connect the interrupted conductor line.
The state-of-the-art structures (1) and (2) can be fabricated by taking advantage of semi-automated techniques for low cost production but they do not provide a low enough resistance in the crossovers for heavy currents. Structure (3) provides the low resistance needed but at the expense of decreased circuit density, lower system reliability, inferior heat dissipation characteristics, and higher production costs.
It is desirable to provide heavy current/high density hybrid circuits using ceramic technology in order to provide an additional improved method of manufacture which maintains both a low material cost and a low labor cost. It is also desirable for such circuits to have low profile crossovers with good heat dissipation characteristics.